1. Field of the Invention
This invention relates to toners used for the dry development of an electrostatic latent image in electrophotography.
2. Description of the Prior Art
There have heretofore been known several dry developing methods including a method using a two-component developer composed of toner particles mixed with carrier particles such as glass beads or magnetic powder, and a method using a one-component toner composed of toner particles imparted with magnetism. In addition, there has recently been proposed a method using a one-component nonmagnetic toner which is excellent in environment resistance, e.g. temperature, humidity and the like.
These conventional toners have been, in most cases, prepared by methods which comprise mixing, heating and melting thermoplastic resins, colorants such as pigments or dyes and additives such as wax, plasticizers, charge-controlling agents and the like, kneading the pigments or charge-controlling agents, changing the form of secondary agglomeration under the application of intense shearing force to primary particles, uniformly dispersing, if necessary, magnetic powder in the mixture to obtain a uniform composition, cooling and comminuting the composition and then classifying the resulting particles to obtain the toner particles.
However, the conventionl methods have problems that they require a large amount of energy in the step of milling the pigment and charge controlling agent, provide the toner particles in a low yield as low as about 85% because finer toner particles are cut or removed in the step of classification, and exhibit an inevitably low productivity of toner particles although part of the removed toner particles are reused in the next production. Further, in cases where particulate toners which are different from each other in kind, particularly hue, are each produced by any conventional method using an apparatus including devices such as a kneader, grinder and dispenser, such devices must beforehand be thoroughly cleaned every time each of the toners is produced. Since the devices used in the conventional methods are considerably large in scale, the cleaning is a heavy burden to the workers.
Moreover, the toner particles so obtained are qualitatively disadvantageous in that they have the charge-controlling agent insufficently dispersed therein and are non-uniform in size and shape and generally amorphous whereby they are individually different in frictional charging characteristics thus causing them to be stained and scattered within the copying devices. In addition, the toner particles have so low flowability that it becomes difficult to supply them smoothly with many troubles being undesirably involved.
To avoid this, there have been proposed attempts to obtain spherical toners by a spray drying or suspension polymerization process. However, the former process requires proper selection of resins which are soluble in the solution used and raises a problem as to an offset phenomenon on a fixing drum. The latter process raises problems as to blocking and offset phenomena and is therefore not industrially used.
Further, other methods proposed include a method in which binder resin particles and coloring material particles are treated in a hot air stream (Japanese patent application Laid-Open No. 37553/1984) and a method in which a binder resin and a coloring material are deposited on the surface of a spherical resin (Japanese patent application Laid-Open No. 210368/1986). However, these methods involving heat treatment tend to cause the formation of coarse particles due to the fusion of fine particles and have not yet been put to practical use.
Conventional toners have the common disadvantage that the colorant and charge controlling agent, which exhibit their characteristic properties on the toner surface and are relatively expensive as starting materials for the toners, are not economically contained not only in the surface portion of the toner but also in the interior.
In order to solve this problem, one of the present inventors proposed a toner produced by mixing a toner for use in electrophotography with a charge controlling agent while applying mechanical strain force to embed the charge controlling agent in the surface of the toner (Japanese patent application No. 51481/1986). However, it was found that, even with this improved method, the desired charge control cannot be often attained depending on the formulation of the toner and the conditions of production. Specifically, when the surface of the core particles (A) is preliminary coated with an electrically conductive material such as carbon black or a surfactant, difficulties are encountered in controlling the charges even when the charge controlling agent (B) is embedded in the surface of the core particles (A) by the above-mentioned method. The reason for this has not yet been fully elucidated. In any way, as is apparent from the foregoing description, the conventional toners still present problems to be solved.